Transducers of the circumferentially magnetised type are disclosed in related U.S. Pat. Nos. 5,351,555 and 5,465,627 (Garshelis, assigned to Magnetoelastic Devices, Inc.) and in U.S. Pat. No. 5,520,059 (Garshelis, assigned to Magnetoelastic Devices, Inc.). These patents describe torque sensing arrangements for a rotating shaft in which a transducer ring or torus is secured to the shaft to rotate therewith and to have the torque developed in the shaft transmitted into the transducer ring. The ring is of a magnetoelastic material circumferentially magnetised and the flux emanating from the ring due to the stress of the ring under torque is detected by a non-contacting sensor system as a measure of the torque.
Another proposal is described in corresponding PCT application PCT/GB99/00736 filed 11th Mar. 1999, published under the number WO99/56099 on 4 Nov. 1999.
In this proposal a shaft of a material capable of exhibiting magnetoelastic material has a portion of it directly magnetised to support a circumferential magnetic field about the shaft axis, an approach which is contrary to the thinking in the prior art. The magnetised portion of the shaft acts directly as the torque transducer element.
To illustrate the operation of a magnetoelastic torque transducer as represented by the above two proposals reference may be made to FIGS. 1A and 1B which show a separate ring transducer element secured to a shaft and a transducer element provided by a portion of the shaft itself. The shafts rotate about the longitudinal axis A-A.
In FIG. 1a the shaft 10, assumed to be of circular cross-section, has the transducer ring 12 securely clamped on it by any of the means described in the three U.S. patents mentioned above. The transducer ring 12 supports a circumferential field Mc extending around the ring. If the shaft 10 is of low magnetic permeability, e.g. paramagnetic, the ring 12 is mounted directly on the outer surface of the shaft. If shaft 10 is ferromagnetic, that is of high permeability, a low permeability spacer (not shown) is mounted between the ring and the shaft.
In FIG. 1b, the solid circular shaft 10′ is of a magnetoelastic material having an integral portion 12′ of it directly circumferentially magnetised to provide the transducer element (the lines delineating portion 12′ are notional for clarity of illustration). As employed in previous torque sensors, with the shaft 10 or 10′ static and no torque applied, the circumferential field Mc is entirely contained within the transducer element 12 or 12′ so that an exterior non-contacting magnetic field sensor will not detect any field emanating from the transducer element. The application of a torque causes the contained field to skew and the opposite sides of the transducer to be oppositely magnetically polarised (i.e. N-S) to generate a torque-dependent magnetic flux that links the poles externally of the transducer element to enable a magnetic sensor to detect the torque-dependent external field Me. It will be understood that this external field forms a torus or doughnut around the shaft. Many different types of magnetic field sensor devices are available and many sensor arrangement configurations of field sensitive devices may be employed. In the instance illustrated in FIG. 1b, the transducer element 12′ is preferably axially bounded by circumferentially-magnetised guard regions 14a and 14b, one to each side. These provide respective poles at the interface that are of opposite polarity, i.e. have a repulsive effect, to the poles of the transducer element enhancing the emanation of externally detectable magnetic field from the transducer element for measurement of torque. The circumferential fields are induced to extend as deeply into the shaft 10′ as possible.
Another proposal to enhance the emanation of magnetic flux from the transducer element is to provide the transducer element portion 12″ shaft 10″ with an integral annular section 16 of raised profile as shown in FIG. 1C, so that the upstanding sides assist in emanating magnetic flux in an external loop. In contrast to the separate ring 12 of FIG. 1 in which all the field is confined in the ring, the annular section 16 is integral and homogeneous with the underlying shaft in FIG. 1c and the circumferential field extends into the body of the shaft. This technique and that of using guard regions is discussed more fully in the aforementioned PCT publication WO99/56099. For the purpose of description of the principles underlying the present invention, no account will be taken of the guard regions where they are used.
Referring to FIGS. 2a and 2b, when the transducer element—12′ is shown as an example, it is equally applicable to separate ring 12—is subject to torque, a magnetic field Me emanates from the transducer for sensing by a sensor arrangement 18 of any desired kind. The external field has a magnitude proportional to the torque and a polarity dependent on the direction of torque T (CW or CCW) as indicated in FIGS. 2a and 2B respectively.
The problem posed to some users of magnetoelastic torque transducer elements of the circumferential magnetic field kind discussed above is that at zero torque in the shaft, the magnetic field output from the transducer element is zero. Outputs at or around the zero region are liable to be masked by noise. Another related problem with circumferential magnetisation is calibrating the transfer function of a transducer, particular in checking the long term calibration where a stored magnetic field may change, usually (weaken) over the long term. A means of checking calibration without a lengthy procedure is a desirable feature. The zero output at zero torque is also a problem with transducers employing longitudinal magnetisation and radially-spaced magnetisation. Longitudinal magnetisation has a detectable axial fringe field at zero torque upon which calibration can be based.